Magnetic encoder technology is used in areas such as motor speed control, robot position control, and various precision rotational measurement instruments. The absolute angular position and number of turns of a shaft are very important control parameters, and therefore the ability to achieve more accuracy while also counting the number of turns is highly desired.
Currently two types of absolute encoder technology are widely used, namely optical encoder technology and a magnetic encoder technology. Optical encoder technology is affected by bubbles, bright light, dirt, leakage and other factors that reduce measurement accuracy. Compared with optical encoder technology, magnetic encoder technology is not affected by these factors, and it has higher resolution, good stability, and can completely eliminate the faults inherent in photovoltaic technology. It is thus a good alternative.
Gear-based multi-turn encoder technology is simple and intuitive, and it has been widely used in multi-turn encoders. Here, the input shaft is connected to an output shaft using a reduction gear, and the speed of rotation of the output shaft is reduced by the reduction gear. This mechanism can be combined with magnetic sensors to measure the turns at each reduction level, and then converted into the number of turns of the input shaft. Assuming a reduction gear ratio of 10:1, 10 rotations of the input shaft yields one rotation of the output shaft. If rotating shafts are divided into 10 equal increments around their circumference, then the higher level shaft moves one increment for each rotation of the lower level shaft, and therefore an absolute position measurement and number of rotations of the input shaft can be obtained. Similarly, a second output shaft may be connected to the first output shaft though a reduction gear, and the second output shaft speed is further reduced. Again, assuming a gear ratio is 10:1 in the second reduction gear set, when the input shaft rotates 100 turns, the first gear shaft rotates 10 turns, and the second gear set rotates one turn. Relying on this mechanical reduction gear mechanism, one can calculate the total number of revolutions of the input shaft. Thus, the number of reduction gear stages directly determines the maximum number of turns of the input shaft that a multi-turn encoder can measure.
Additionally, magnetic absolute encoder technology measurement accuracy depends on the performance characteristics of the magnetoresistive angle sensors and the permanent magnet design. Compared with Hall sensors, magnetoresistive sensors such as tunneling magnetoresistive sensors have better magnetic field sensitivity, lower power consumption, and smaller size. A tunneling magnetoresistive angular displacement sensor may comprise two mutually orthogonal tunneling magnetoresistive sensors. The tunneling magnetoresistive angular displacement sensor produces two outputs from the magnetic field of the rotating permanent magnet, representing the sine and the cosine of the orientation angle φ of the magnetic field generated by the permanent magnet, and these components can be used to calculate the angle using the following relationships:OUT1=COS(φ)OUT2=SIN(φ)
The inverse tangent function can then be used to calculate the angle φ of the rotating magnetic field from the magnetoresistive angular displacement sensor outputs OUT1 and OUT2:φ=A TAN(OUT2/OUT1).
When the permanent magnet rotates by angle α, the magnetic field produced by the permanent magnet passes through and is detected by the tunneling magnetoresistive sensors located a point defined by r and an angle. When the angle of the magnet α and the angle of the magnetic field φ form a linear relationship in the range of 0˜360°, then the angle φ of the magnetic field detected by the tunneling magnetoresistive sensor represents the mechanical angle α of the permanent magnet, which represents the mechanical rotation angle of the shaft.
Thus, tunneling magnetoresistive angular displacement sensors will have special requirements for the design of the permanent magnet when applied to multiturn absolute magnetic encoders, but these sensors are better than those used in existing permanent magnet based multiturn absolute magnetic encoder s. Existing magnetic field sensor based absolute encoders have the following disadvantages and complications:
(1) The existing magnetic absolute encoder technology uses a Hall sensor as the magnetic field angle measurement devices, and they therefore have high power consumption and low resolution.
(2) Existing magnetic angle encoders using Hall sensors must detect the perpendicular magnetic field component perpendicular to the sensor surface generated by the permanent magnet, and tunneling magnetoresistive sensors detect the magnetic field component parallel to the surface of the sensor, so existing permanent magnet designs are not compatible with the tunneling magnetoresistive sensors.
(3) Existing permanent magnet based absolute encoders generally use a solid cylindrical permanent design, whereby the permanent magnet is fixed on the ends of the shafts, which increases space, while the cylindrical ring magnet design can be mounted directly on or into a counting wheel in order to make a more compact design.